The present invention relates to a method for polishing a wafer and an apparatus for polishing a wafer, and furthermore to a wafer holding plate, a wafer adhering method and a wafer adhering apparatus that are used for the method and apparatus for polishing a wafer.
Reflecting increasing a diameter of a silicon wafer and improving of high accuracy of a device fabricated by employing it, the requirements for final accuracy (thickness uniformity, flatness and smoothness) of a silicon wafer (polished wafer) which is polished for finish are becoming higher.
In order to satisfy such requirements, techniques of polishing process for wafer have been improved, and apparatuses for the polishing process have been developed and improved.
As one of them, so-called single wafer polishing apparatuses have been freshly developed for the purpose of polishing a wafer having a large diameter of, particularly 300 mm or mote, and some of them are put to practical use.
However, with the single wafer polishing method, problems that (1) is difficult to meet the requirement for a reduction of wafer costs in view of productivity, (2) it is difficult to meet the recent requirement for flatness in the vicinity of peripheral edge of the wafer (within 1 mm from peripheral outer edge), or the like have arisen.
On the other hand, with a polishing method by a batch processing polishing apparatus in which a plurality of wafers are adhered to a wafer holding plate with adhesive, and then polished simultaneously, which is popularized most, the constitution and structures of the apparatus and performance thereof are being improved for achieving She high accurate finish. However, there is the following problem according to the adhesion mounting of the wafer on the wafer holding plate via adhesive.
That is, although ceramic materials have employed for the wafer holding plate, for example, with a wafer holding plate made of sintered ceramic including alumina as the main component, thickness unevenness of an adhesive layer generates in an adhesion portion. It is considered that the thickness unevenness is caused by surface roughness and fine structure of the wafer holding plate, physical property of an interface between the adhesive and the ceramic, or the like. The thickness unevenness causes unfavorable ununiformity of the thickness in the wafer after polishing.
On the other hand, with a wafer holding plate made of glass, the above-described problem of the thickness unevenness of the adhesive layer in the adhesion portion is avoided. However, when the wafer is pressed against a polishing pad during polishing, the glass bends, causing excess polishing of the peripheral portion of the wafer compared to other portions. Thus, the flatness of the wafer lowers.
Further, when the wafer is adhered to the wafer holding plate, very small amount of air remains between the wafer holding plate and the wafer. A portion of a surface to be polished of the wafer, which is corresponding to a portion sandwiching the remained air bubble, protuberates slightly, and is polished extra. As a result, the thickness unevenness of the wafer and the lowering of the flatness of the polished surface are caused.
In order to solve this problem, a jig that curves the wafer so that the air between the wafer and the wafer holding plate will be let out from the center of the wafer toward the periphery of the wafer, and that is for adhering the wafer to the wafer holding plate from the center of the wafer has been developed. However, with the jig having the previous structure, a surface to be adhered of the wafer does not become a uniform convex surface, but a part of the surface to be adhered forms a concave surface portion. Thereby, the air remains in the concave surface portion, and thus the problem is not solved completely.
Furthermore, particularly, with the wafer holding plate used for the batch processing polishing apparatus, since the wafer is adhered to the wafer holding plate at an eccentric position, even if the wafer holding plate is rotated inductively, rubbing rates vary in-plane in a single wafer against the polishing pad. Thus, the peripheral portion of the wafer is excessively polished compared to the central portion thereof. This is a factor obstructive to the improvement of the flatness of the wafer.
For solving the problems caused by the adhesion of the wafer described above, investigations and trial manufactures were repeatedly carried out eagerly. Then, it was achieved developing an adhering method and an adhering apparatus that let out the air between the wafer and the wafer holding plate during the adhesion by forming the surface to be adhered of the wafer into the convex surface as a whole, and then do not make an air bubble remain.
With the adhering method and the adhering apparatus, the wafer is held by vacuum-chucking the surface to be polished of a wafer such that a convex surface is formed in the vicinity including an arbitrary point within a region surrounding the center of the surface to be adhered of the wafer, and the region being at least not less than 50% of the entire adhesion area, and the wafer is adhered to the wafer holding plate from the central portion of the surface to be adhered of the wafer.
When the wafer is curved so as to form such convex surface, it is preferable that a wafer contacting region of a wafer holding surface of a contacting member has a value of radius of curvature between 5 m and 1000 m, the radius of curvature being for a line of intersection between a plane which includes a normal line of an arbitrary point and which passes through a center of the wafer contacting region and the wafer contacting region, within a region surrounding the center of the wafer contacting region and corresponding to at least not less than 50% of the entire adhesion area of the wafer. Furthermore, it is preferable that the surface to be adhered of the wafer is made such that a value of radius of curvature of a line of intersection between a plane including a normal line of an arbitrary point and passing through a center of the surface to be adhered and the surface to be adhered is between 5 m and 1000 m, within a surface which surrounds the center of the surface to be adhered and which is at least not less than 50%. The reason therefor is that when the value of radius of curvature is less than 5 m, problems that {circle around (1)} when the vacuum-chuck is released, because a restoring force of the wafer is large, the peripheral portion of the wafer is immediately brought into contact with the wafer holding plate, thereby the air between the wafer holding plate and the wafer is not removed sufficiently, {circle around (2)} excessive stress generates inside of the wafer, {circle around (3)} it is required to increase the vacuum-sucking force for the wafer, or the like arise. On the other hand, the reason therefor is that when the value of the radius of curvature is more than 1000 m, there are problems that the peripheral portion of the wafer is brought into contact with the wafer holding plate before the air in the center lets out, thereby the air between the wafer holding plate and the wafer is not removed sufficiently, or the like.
As a concrete method for forming the surface to be adhered of the wafer into the above-described convex surface, the wafer holding surface itself of the contacting member chucking the surface to be polished of the wafer may be formed into the convex surface, or the contacting member chucking the surface to be polished of the wafer may be transformed by a pressure applying means such that the surface to be adhered of the wafer forms the convex surface. In the latter case, for example, an air supply and discharge apparatus is used as the pressure applying means.
On the other hand, at first, the wafer can be held by vacuum-chucking only in the peripheral portion of the surface to be polished of the wafer. In this case, it may be preferable that the wafer may be held by vacuum-chucking with a plurality of holders having sucking disk structure. When the vacuum-chuck holding is carried out with the plurality of holders, each chucked area can be reduced all the better for that. Thus, no concave surface portion generates in the surface to be adhered of the wafer.
Alternatively, holding of the wafer by vacuum-chucking may be carried out by vacuum-sucking the surface to be adhered of the wafer through a small hole for discharging air, the small hole being provided to the contacting member, or by vacuum-sucking the surface to be adhered of the wafer through a groove formed in the wafer holding surface of the contacting member.
The contacting member may be made of ceramic, polymeric material or metallic material, for example. When the metallic material is employed, at least a wafer holding surface of the contacting member is coated with nonmetallic material.
The inventors made various developments and trial manufactures for solving the thickness unevenness of the adhesive layer in the adhesion portion when the wafer is adhered to the wafer holding plate made of ceramic. The inventors developed a wafer holding plate that is capable of combining rigidity of ceramic and an excellent adhesion property of the wafer holding plate made of glass, and then achieved obtaining a high accurate polished wafer by applying a polishing method and a polishing apparatus to which these development results are adapted.
For example, a base member of the wafer holding plate is made of ceramic and a surface for adhering a wafer comprises a glass layer. Alternatively, the base member is made of ceramic and a side for adhering a wafer is provided with a glass plate. In this case, it is preferable that the glass comprises borosilicate glass. Furthermore, it is preferable that the glass layer has a thickness between 0.05 mm and 2 mm, and the glass plate has a thickness between 1 mm and 20 mm.
Further, adhering the wafer to the wafer holding plate combining the rigidity and the adhesion property by using the improved adhering method and adhering apparatus, and then polishing the wafer has a remarkable effect on improvement of the flatness of the wafer.
Furthermore, when a wafer holding portion of a wafer holding plate employed for the batch processing polishing apparatus is turnable with respect to a base body, the surface to be polished of the wafer comes more uniformly in contact with the polishing pad. Therefore, it has much effect on improvement of the flatness.
When the wafer is adhered to the wafer holding plate, it is preferable that a wafer is adhered while holding the wafer such that difference of elevation of the surface to be adhered of the wafer is between 20 xcexcm and 1000 xcexcm. The reason therefor is that when the difference of elevation is more than 1000 xcexcm, problems that {circle around (1)} when the vacuum-chuck is released, because a restoring force of the wafer is large, the peripheral portion of the wafer is immediately brought into contact with the wafer holding plate, thereby the air between the wafer holding plate and the wafer is not removed sufficiently, {circle around (2)} excessive stress generates inside of the wafer, {circle around (3)} it is required to increase the vacuum-sucking force for the wafer, or the like arise. On the other hand, the reason therefor is that when the difference of elevation is less than 20 xcexcm, there are problems that the peripheral portion of the wafer is brought into contact with the wafer holding plate before the air in the center lets out, thereby the air between the wafer holding plate and the wafer is not removed sufficiently, or the like.